Microwave discharges are suitably used as microwave power light sources. The present invention is particularly suited to light sources. In a first instance the light source can be a source of incoherent optical radiation extending over a wide range, as for example a significant portion of the ultraviolet, visible, or infrared band. In a second instance the light source can be a laser. The laser may emit coherent radiation at one or plural discrete wavelengths.
Microwave lamps generally comprise a cavity formed with at least a wall surface which is substantially open mesh. A discharge envelope is provided in the cavity. A means for transferring microwave power to the cavity comprises, for example, a waveguide and a slot. A magnetron is connected via the transferring means to the cavity. Microwave energy on the order of a hundreds to thousands of watts is coupled to the cavity. The envelope contains a discharge medium, which typically includes mercury and a rare gas.
While a laser is a different type of discharge device than a lamp, a laser may also be made using a microwave powered discharge. An elongated discharge tube is disposed in relation to a means for charging the tube with microwave energy. The tube may be provided with Brewster windows at its ends to polarize the light produced by the laser. A fully reflective mirror, and a partially reflective mirror are typically located facing the ends of the tube to form an optical resonator.
For the microwave lamps and lasers for which the present invention is particularly useful, it is desirable to excite an elongated or linear discharge.
An article by Stephan Offermanns, Journal of Applied Physics, 67(1), 1 Jan. 1990, Pp. 115-123 discusses the use of a cavity in which the electric field is parallel to an elongated discharge envelope. The cavity disclosed by Offermanns is the right cylinder TM.sub.010. If optimized for operation at 2.45 GHz which is the ISM band used in industrial microwave lamps, this cavity would measure 3.6" in diameter. Since the envelope or bulb is located on the axis of the cavity, it would be disposed within 1.6" of the coupling means, which is located at the cavity wall. At industrially useful power levels i.e. hundreds to thousands of watts, the close proximity of the coupling means to the envelope would tend to cause direct coupling from the coupling means to the discharge envelope. Such direct coupling is not uniform, but rather is strongest nearest to the slot.
Principles of microwave engineering can be used to predict a certain field distribution in a particular cavity operating in a particular mode. The inventors have discovered that in practice the actual field measured in the cavity has a component which is not accounted for according to the predicted field distribution. Additionally, the inventors have ascribed the anomalous component of the field to the radiation field directly from the coupling means. The severity of this phenomenon appears to be inversely related to the distance between the bulb and the coupling means. In summary, in microwave discharge devices that use relatively resonant cavities, there is a problem in that the coupling means, (e.g. slot) couples directly to the bulb, rather than coupling to the oscillation mode in the cavity, which then couples to the bulb. Direct coupling from the slot results in sharp peaks in discharge intensity closest to the slot.